Cant get rid of fin rot, help?

[Marne]

Have treated my tank several times for fin rot over the past 2 months, to begin with it was just a couple of my young guppies, treated the tank and then waited a week and there was no improvement so I treated again, now a few more guppies have it too and I have lost a swordtail which had it, none of my mollies, plec's or corys are effected just seems to be the guppies (and the one swordtail). I really don't want to treat the tank again as it seems to effect my panda corys, is there anything else I cad do?

What sort of % water changes are you doing and what are your ammonia/nitrate/nitrite levels like?

doing 30% change weekly (sometimes more), water tests are fine other than high (ish) ph. Tank is 200ltr community tank, have always had the odd death probably due to high ph but have lowered it slightly since having bog wood in the tank, just seems to be on generation of my guppy fry that are effected. I'm worried about treating again because of my corys and I have molly fry (and my survival rate for molly fry is rubbish, would love for these to survive).

Any way you could quickly set up a little quarantine tank and whip the affected ones in to it? That way you can treat the infection locally without messing with the other fish.
To lower your ph you could get some RO water from your local fish shop.

I havn' got another tank

Then I think you're not left with much choice but to keep dosing the tank. Or try to get a cheapy tank from eBay/Preloved/freecycle in the next day or so.
What temperature are you keepin the tank at? If the fish will be ok try upping it by one or two degrees when you next dose it.

Tank temp is around 24 (I think, must check), hopefully my swordtail will give birth today, she's looking huge, I don't really want to treat when she's in labour , one of the guppies that have the fin rot died last night, in a way I am pleased as now I only have 2 or 3 with it. Will the treatment effect my fry? I don't want to kill my baby mollies, I only have 4 .